Thermal spray deposition of hollow microspheres

ABSTRACT

Methods of forming an insulating coating from thermal spraying are provides. In one variation, the method includes thermally spraying a jetted stream having a maximum temperature of greater than or equal to about 900° C. towards a substrate to form the insulating coating on the substrate. The thermal spraying may be a high-velocity oxygen flame (HVOF) process. The jetted stream comprises a plurality of hollow microspheres, which may comprise a metal, such as nickel or iron. The insulating coating as formed has a thermal conductivity (K) of less than or equal to about 200 mW/m·K at standard temperature and pressure conditions and may have a thermal capacity (c v ) of greater than or equal to about 100 kJ/m 3 ·K.

INTRODUCTION

This section provides background information related to the presentdisclosure which is not necessarily prior art.

The present disclosure pertains to methods for thermally spraying hollowmicroparticles onto substrates to form insulating thermal barriercoatings.

Insulating and thermal barrier coatings are used in various applicationsto reduce heat transfer. Such coatings desirably have low heat capacityand low thermal conductivity. In certain aspects, a thermal barriercoating may comprise an insulating material including one or more hollowmicrospheres. Thus, the insulating or thermal barrier coating can beused in a variety of applications, including by way of non-limitingexample, on surfaces of components within an internal combustion engineto reduce heat transfer losses and increase performance and efficiency.New methods of forming robust thermal barrier coatings on a variety ofcomplex components are desirable.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

The present disclosure relates to thermal spray deposition of hollowmicrostructures, such as microspheres. In one variation, the presentdisclosure provides a method of forming an insulating coating includingthermally spraying a jetted stream having a maximum temperature ofgreater than or equal to about 900° C. towards a substrate to form theinsulating coating on the substrate. The jetted stream includes aplurality of hollow microspheres. The insulating coating that is formedhas a thermal conductivity (K) of less than or equal to about 200 mW/m·Kat standard temperature and pressure conditions.

In one aspect, the insulating coating includes a plurality of hollowmicrostructures having intact void regions after the thermal spraying.

In another aspect, the insulating coating has a net porosity of greaterthan or equal to about 80 volume %.

In yet another aspect, the insulating coating has a thickness of lessthan or equal to about 200 micrometers (μm).

In certain aspects, the jetted stream has a maximum temperature of lessthan or equal to about 1,400° C.

In another aspect, the plurality of microspheres includes a metalselected from the group consisting of: nickel, iron, combinations, andalloys thereof.

In other aspects, the plurality of microspheres includes the metal in afirst layer and further includes a second layer of a second metalselected from the group consisting of: copper, zinc, tin, nickel, andcombinations thereof.

In yet another aspect, the substrate includes at least one metalselected from the group consisting of: nickel, iron, copper, zinc,aluminum, combinations, and alloys thereof.

In another aspect, the thermal conductivity (K) of the insulatingcoating is less than or equal to about 100 mW/m·K at standardtemperature and pressure conditions.

In further aspects, a thermal capacity (c_(v)) of the insulating coatingis less than or equal to about 100 kJ/m³·K.

In another variation, the present disclosure provides a method offorming an insulating coating including jetting a stream including aplurality of hollow microspheres from a high velocity oxygen fuel (HVOF)device towards a substrate. The stream includes the plurality ofmicrospheres that include a first metal layer having a first metalselected from the group consisting of: nickel, iron, combinations, andalloys thereof and a second metal layer having a second metal selectedfrom the group consisting of: copper, zinc, tin, nickel, combinations,and alloys thereof. Further, the stream has a maximum temperature duringthe jetting that is at least about 50° C. below a melting point of thefirst metal layer, but at or above a melting point of the second metallayer. The method also includes forming the insulating coating on thesubstrate having a thermal conductivity (K) of less than or equal toabout 200 mW/m·K at standard temperature and pressure.

In one aspect, the insulating coating includes a plurality of hollowmicrostructures having intact void regions after the thermal spraying.

In another aspect, the insulating coating has a net porosity of greaterthan or equal to about 80 volume %.

In yet another aspect, the insulating coating has a thickness of lessthan or equal to about 200 micrometers (μm).

In yet other aspects, the insulating coating may have a thickness ofless than or equal to about 200 micrometers (μm)

In a further aspect, the maximum temperature is greater than or equal toabout 900° C. to less than or equal to about 1,400° C.

In yet another aspect, the substrate includes at least one metalselected from the group consisting of: nickel, iron, copper, zinc, tin,nickel, aluminum, combinations, and alloys thereof.

In other aspects, the substrate includes a first metal selected from thegroup consisting of: nickel, iron, combinations, and alloys thereof andfurther includes a surface coating of a second metal selected from thegroup consisting of: copper, zinc, tin, nickel, combinations, and alloysthereof.

In yet other aspects, the plurality of microspheres includes the firstmetal layer having nickel and the second metal layer having copper.

In still further aspects, the thermal conductivity (K) is less than orequal to about 100 mW/m·K at standard temperature and pressureconditions.

In another aspect, the insulating coating has a thermal capacity (c_(v))of less than or equal to about 100 kJ/m³·K.

In yet another aspect, the method further includes sintering theinsulating layer after the jetting.

In yet another variation, the present disclosure provides a method offorming an insulating coating that includes jetting a stream including aplurality of hollow microspheres from a high velocity oxygen fuel (HVOF)device towards a substrate to form a layer of deposited hollowmicrostructures. The stream has a maximum temperature during the jettingthat is greater than or equal to about 900° C. to less than or equal toabout 1,400° C. Each of the plurality of hollow microspheres includes afirst metal layer and a second metal layer. The first metal layer has afirst metal selected from the group consisting of: nickel, iron,combinations, and alloys thereof and the second metal layer has a secondmetal selected from the group consisting of: copper, zinc, tin, nickel,combinations, and alloys thereof. The method further includes sinteringthe layer of deposited hollow microstructures to form the insulatingcoating on the substrate having a thermal conductivity (K) of less thanor equal to about 200 mW/m·K at standard temperature and pressureconditions and a thermal capacity (c_(v)) of less than or equal to about100 kJ/m³·K.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 shows a high velocity oxygen flame (HVOF) thermal spraying devicethat can be used in accordance with certain aspects of the presentdisclosure for depositing hollow microspheres to form an insulatingcoating.

FIG. 2 shows an example of a hollow microsphere having a single metalcoating layer.

FIG. 3 shows another example of a hollow microsphere having two distinctmetal coating layers for use in the thermal spraying processes accordingto certain aspects of the present disclosure.

FIG. 4 shows an insulating coating including hollow microstructuresdeposited on a substrate in accordance with certain aspects of thepresent disclosure via thermal spraying.

FIG. 5 shows a composite thermal barrier coating including an insulatingcoating including hollow microstructures deposited on a substrate inaccordance with certain aspects of the present disclosure via thermalspraying.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION

Example embodiments are provided so that this disclosure will bethorough, and will fully convey the scope to those who are skilled inthe art. Numerous specific details are set forth such as examples ofspecific compositions, components, devices, and methods, to provide athorough understanding of embodiments of the present disclosure. It willbe apparent to those skilled in the art that specific details need notbe employed, that example embodiments may be embodied in many differentforms and that neither should be construed to limit the scope of thedisclosure. In some example embodiments, well-known processes,well-known device structures, and well-known technologies are notdescribed in detail.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a,” “an,” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The terms “comprises,” “comprising,” “including,” and“having,” are inclusive and therefore specify the presence of statedfeatures, elements, compositions, steps, integers, operations, and/orcomponents, but do not preclude the presence or addition of one or moreother features, integers, steps, operations, elements, components,and/or groups thereof. Although the open-ended term “comprising,” is tobe understood as a non-restrictive term used to describe and claimvarious embodiments set forth herein, in certain aspects, the term mayalternatively be understood to instead be a more limiting andrestrictive term, such as “consisting of” or “consisting essentiallyof.” Thus, for any given embodiment reciting compositions, materials,components, elements, features, integers, operations, and/or processsteps, the present disclosure also specifically includes embodimentsconsisting of, or consisting essentially of, such recited compositions,materials, components, elements, features, integers, operations, and/orprocess steps. In the case of “consisting of,” the alternativeembodiment excludes any additional compositions, materials, components,elements, features, integers, operations, and/or process steps, while inthe case of “consisting essentially of,” any additional compositions,materials, components, elements, features, integers, operations, and/orprocess steps that materially affect the basic and novel characteristicsare excluded from such an embodiment, but any compositions, materials,components, elements, features, integers, operations, and/or processsteps that do not materially affect the basic and novel characteristicscan be included in the embodiment.

Any method steps, processes, and operations described herein are not tobe construed as necessarily requiring their performance in theparticular order discussed or illustrated, unless specificallyidentified as an order of performance. It is also to be understood thatadditional or alternative steps may be employed, unless otherwiseindicated.

When a component, element, or layer is referred to as being “on,”“engaged to,” “connected to,” or “coupled to” another element or layer,it may be directly on, engaged, connected or coupled to the othercomponent, element, or layer, or intervening elements or layers may bepresent. In contrast, when an element is referred to as being “directlyon,” “directly engaged to,” “directly connected to,” or “directlycoupled to” another element or layer, there may be no interveningelements or layers present. Other words used to describe therelationship between elements should be interpreted in a like fashion(e.g., “between” versus “directly between,” “adjacent” versus “directlyadjacent,” etc.). As used herein, the term “and/or” includes any and allcombinations of one or more of the associated listed items.

Although the terms first, second, third, etc. may be used herein todescribe various steps, elements, components, regions, layers and/orsections, these steps, elements, components, regions, layers and/orsections should not be limited by these terms, unless otherwiseindicated. These terms may be only used to distinguish one step,element, component, region, layer or section from another step, element,component, region, layer or section. Terms such as “first,” “second,”and other numerical terms when used herein do not imply a sequence ororder unless clearly indicated by the context. Thus, a first step,element, component, region, layer or section discussed below could betermed a second step, element, component, region, layer or sectionwithout departing from the teachings of the example embodiments.

Spatially or temporally relative terms, such as “before,” “after,”“inner,” “outer,” “beneath,” “below,” “lower,” “above,” “upper,” and thelike, may be used herein for ease of description to describe one elementor feature's relationship to another element(s) or feature(s) asillustrated in the figures. Spatially or temporally relative terms maybe intended to encompass different orientations of the device or systemin use or operation in addition to the orientation depicted in thefigures.

Throughout this disclosure, the numerical values represent approximatemeasures or limits to ranges to encompass minor deviations from thegiven values and embodiments having about the value mentioned as well asthose having exactly the value mentioned. Other than in the workingexamples provided at the end of the detailed description, all numericalvalues of parameters (e.g., of quantities or conditions) in thisspecification, including the appended claims, are to be understood asbeing modified in all instances by the term “about” whether or not“about” actually appears before the numerical value. “About” indicatesthat the stated numerical value allows some slight imprecision (withsome approach to exactness in the value; approximately or reasonablyclose to the value; nearly). If the imprecision provided by “about” isnot otherwise understood in the art with this ordinary meaning, then“about” as used herein indicates at least variations that may arise fromordinary methods of measuring and using such parameters. For example,“about” may comprise a variation of less than or equal to 5%, optionallyless than or equal to 4%, optionally less than or equal to 3%,optionally less than or equal to 2%, optionally less than or equal to1%, optionally less than or equal to 0.5%, and in certain aspects,optionally less than or equal to 0.1%.

In addition, disclosure of ranges includes disclosure of all values andfurther divided ranges within the entire range, including endpoints andsub-ranges given for the ranges.

Example embodiments will now be described more fully with reference tothe accompanying drawings.

In various aspects, the present disclosure describes methods of formingan insulating coating on a substrate. In certain aspects, the method mayinclude thermally spraying a jetted stream towards a substrate. Bythermal spraying, it is meant that a process is employed where aprecursor material is heated and propelled as individual particles at asurface of the substrate to form a robust and cohesive coating. Thermalspraying thus relies on heat and momentum to cause the coating materialto conform and bond to the surface being coated.

In certain aspects, the thermal spraying process has a maximumtemperature of greater than or equal to about 900° C., optionallygreater than or equal to about 1,000° C., optionally greater than orequal to about 1,100° C., and in certain aspects, optionally greaterthan or equal to about 1,200° C. In certain aspects, the thermalspraying process has a temperature of greater than or equal to about900° C. to less than or equal to about 1,400° C., and in certainvariations, optionally greater than or equal to about 1,100° C. to lessthan or equal to about 1,200° C. As will be appreciated by those ofskill in the art, some cooling occurs as the material being sprayedexits the thermal spraying device or gun, thus desirably the temperaturein the thermal spray device is high enough to promote softening ormelting of at least one material in a hollow precursor, while avoidingoverheating that could cause structural collapse, as will be describedfurther below.

For all thermal spray coating processes, material is heated,accelerated, and shot at a target surface. Particle velocities vary indifferent thermal spray processes, for example, velocities of jettedstreams are highest in high-velocity oxygen flame processes and lower inlow velocity spraying processes, such as subsonic oxygen fuel powderprocesses. By a jetted stream, it is meant that the stream has arelatively high velocity and creates a jet, while still desirablyavoiding velocities that would promote collapse of a hollow precursorunder the select conditions used during thermal spraying (e.g., selecttemperatures and pressures). For example, a maximum velocity of a jettedstream for use in accordance with certain aspects of the presentdisclosure is less than or equal to about 400 m/s, optionally less thanor equal to about 100 m/s, and in certain aspects, optionally less thanor equal to about 10 m/s. In certain variations, the jetted stream mayhave supersonic velocity of greater than about 343 m/s but less than orequal to about 400 m/s.

With the high-velocity oxygen flame coating process, the gas stream isproduced by mixing and igniting oxygen and fuel (gas or liquid) in acombustion chamber and allowing the high pressure gas to acceleratethrough a nozzle. Microparticles are introduced into this stream whereit is heated and accelerated towards a target surface. In one variationaccording to certain aspects of the present disclosure, a method offorming an insulating coating is shown in FIG. 1 that includes jetting astream comprising a plurality of hollow microparticles or microspheres102 from a high velocity oxygen fuel (HVOF) device 100 towards asubstrate 110. The hollow microparticles define an enclosed void regionin the core that may be filled with an insulating material. Thus, thevoid region in the core may be filled with a gas, such as air or aninert gas, or may have vacuum conditions, by way of example. In certainaspects, the microparticles have at least one spatial dimension that isless than about 100 μm optionally less than or equal to about 50 μm andin certain aspects, less than or equal to about 10 μm.

The microparticles may be microspheres having a substantially roundshape. “Substantially round-shaped” includes microparticles having ashape including spherical, globular, spheroidal, disk, cylindrical,discoid, domical, egg-shaped, elliptical, orbed, oval, and the like, solong as at least a portion of the center of the microparticle defines anenclosed void region. Thus reference herein to microspheres canencompass any of these substantially round shapes. Notably, while theprecursor may be a microsphere, after the thermal spraying process, themicrosphere may be distorted from a spherical or substantially roundshape.

In certain aspects, the microspheres used as a precursor during thethermal spraying may have an average particle size diameter of less thanabout 100 micrometers (μm), optionally greater than or equal to about 10μm to less than or equal to about 80 μm, optionally greater than orequal to about 20 μm to less than or equal to about 60 μm, and incertain variations, optionally greater than or equal to about 30 μm toless than or equal to about 40 μm. It should be appreciated that themicrospheres have an average diameter within these ranges, but theplurality of microspheres do not necessarily all have the same diameter,as a mixture of microspheres having distinct diameters may be employedto provide a desired porosity or packing density, which can varystrength within the insulating coating formed. It should be furthernoted that the smaller the diameter of the microspheres used, thegreater the particle density and therefore mass of the coating formedfrom such particles. Thus, relatively large microspheres form lightercoatings than smaller microspheres.

The plurality of microspheres comprises at least one metal. In certainvariations, the metal may be selected from the group consisting of:nickel, iron, combinations, and alloys thereof. In one variation, themetal is nickel or a nickel alloy. In another variation, the metal isiron and microsphere may comprise an iron alloy, such as steel orstainless steel. Any alloy may contain additional elements asappreciated by those of skill in the art, such as carbon, manganese,chromium and nickel, molybdenum, and the like, by way of non-limitingexample.

Generally, hollow microspheres 20, such as that shown in FIG. 2, mayinclude a structural material 22 that defines the enclosed void region24. The structural material 22 may be selected from the group consistingof: metal, glass, ceramic, polymers, and combinations thereof, so longthe structural material 24 defines a hollow structure having theenclosed void region 24. The hollow microspheres 20 may then be coatedwith one or more conductive materials, such as metals like nickel, iron,nickel alloy compounds, iron alloy compounds, and the like. A metalcoating 26 is thus formed over the structural material 22. Theapplication of the metal on the surface of the microspheres 20 may beconducted via a process such as electroplating, vapor deposition,electroless plating, flame spraying, painting, and the like. Themicrosphere 20 may be used in thermal spraying, but in certain aspectsis used as a precursor for forming a multilayered hollow microspherelike that shown in FIG. 3.

In FIG. 3, a hollow microsphere 30 used as a precursor in the thermalspraying process according to certain aspects of the present disclosuremay have multiple distinct metallic layers. In one variation, the hollowmicrospheres 30 have a structural material 32 that defines a void region34 in the center core region. The structural material 32 may be formedof the same materials as structural material 22 above. A first metallicmaterial comprising a first metal may be applied to the surface of thestructural material 32 to form a first metal layer or coating 36. Asecond metallic material may comprise a second distinct metal that bemay applied over the first metal coating 36 to form a second metal layeror coating 38.

The first metal coating 36 may comprise nickel, iron, combinations, andalloys thereof. In certain aspects, the first metal coating 36 comprisesnickel or a nickel alloy. The second metal coating 38 may comprisecopper, zinc, tin, nickel, combinations, and alloys thereof. Notably,while the first metal coating 36 and the second metal coating 38 maycontain one or more of the same metals, each layer/coating has adistinct composition and thus distinct melting points. In certainaspects, the second metal coating 38 comprises copper or a copper alloy.In other aspects, the second metal coating 38 may comprise a combinationof copper and zinc, for example, a brass alloy. In certain aspects, abrass alloy has zinc present in the composition at less than or equal toabout 32% by weight (to avoid formation of undesirable phases), whilethe balance may include copper and impurities. In this manner, the firstmetal coating 36 and second distinct metal coating 38 have differentmelting point temperatures, which can be beneficial for certain thermalspraying processes as described further below. More specifically, thefirst metal layer or coating 36 may have a higher melting point than thesecond metal layer or coating 38. Thus, in certain variations, thesecond metal coating 38 may comprise copper and nickel, where nickel ispresent in the composition up to about 30% by weight and the balance iscopper and impurities. In other aspects, the second metal coating 38 maycomprise nickel and tin, where tin is present in the composition up toabout 30% by weight and the balance is nickel and impurities. Such anickel-tin alloy has a low melting point/eutectic point of about 1130°C. In certain other aspects, the second metal coating 38 may comprisenickel and zinc, where zinc is present in the composition up to about40% by weight and the balance is nickel and impurities.

In certain aspects, the first metal coating 36 may have a thickness ofabout 1 micrometer, while the second metal coating 38 may have athickness of less than or equal to about 1 micrometer. Thus, thethickness of the second metal layer or coating 38 is less than thethickness of the first metal layer or coating 36. Where the first metalcoating 36 comprises nickel and the second metal coating 38 comprisescopper, the copper can diffuse into the nickel. The more copper thatdiffuses from the second metal coating 38 into the first metal coating36, the lower the maximum temperature that can be used while thermallyspraying while still maintaining the hollow structures (e.g., so thatthe first metal coating 36 remains structurally intact as a hollowshape, especially when the structural material 32 has been removed fromthe hollow microsphere 30). However, most diffusion of copper intonickel takes place after the thermal spray process, if a subsequent heattreatment (e.g., for sintering) is conducted. The final alloycomposition (the Ni—Cu alloy) can potentially limits a maximumtemperature of use in the final application (e.g., as a thermal barriermaterial in an engine).

With renewed reference to FIG. 1, the HVOF process uses combustion togenerate heat and impart velocity to the jetted stream. The HVOF device100 includes two fuel inlets 120 for a fuel stream and two oxidantinlets 122 for introducing an oxygen-containing stream (or otheroxidant-containing stream). It should be noted that the fuel inlets 120and oxidant inlets 122 are not restricted to the number of inlets orplacement shown in FIG. 3. The fuel may comprise propane, propylene,and/or hydrogen, by way of non-limiting example. Further, while notshown, cooling inlets and channels that circulate a coolant may also beprovided. A central inlet 130 receives a stream of a carrier gas and theplurality of hollow microspheres 102 insufflated therein. The carriergas may be an inert gas, such as nitrogen and/or argon, by way ofnon-limiting example, or the carrier gas may have the same compositionas the oxidant-containing stream or fuel stream. In certain alternativedesigns not shown, the central inlet 130 could be eliminated or modifiedso that the hollow microspheres are directly introduced into the fuelstream(s) or oxidant stream(s).

In the HVOF device 100, the oxygen-containing stream and fuel combine ina mixing region 132. The mixed stream is introduced into a combustionchamber 134 where an exothermic combustion reaction takes place. Thecarrier gas and hollow microspheres 102 pass into and through thecombustion chamber 134 where they are heated and then enter a nozzle136. A high temperature, high velocity jetted stream 140 exits thenozzle 136. The jetted stream 140 may be a supersonic spray flame incertain variations. The parameters of operation for the HVOF device 100are selected in accordance with certain aspects of the presentdisclosure to promote softening, adhesion, and bonding of themicrospheres onto the substrate, while minimizing collapse or rupture ofthe interior void regions.

For example, in certain aspects, a maximum temperature of the jettedstream 140 (and within the HVOF device 100) is selected to be at leastabout 50° C. below a melting point of a select metal forming the hollowmicrospheres 102, for example, a first metal in the first metallayer/coating. Thus, in certain aspects, where the metal is nickel, amaximum temperature is at least 50° C. below the melting temperature ofnickel of 1,455° C., so that a maximum temperature in the HVOF processand the jetted stream is less than about 1,405° C. (or about 1,400° C.).In certain variations, throughout the process, the stream (andmicrospheres) encounter a maximum temperature during the jetting that isgreater than or equal to about 900° C. to less than or equal to about1,400° C.; optionally greater than or equal to about 1,000° C. to lessthan or equal to about 1,300° C., and in certain aspects, optionallygreater than or equal to about 1,100° C. to less than or equal to about1,200° C. The operating temperature and pressures used in HVOF can betuned to allow the deposition of various microspheres, such as thosecomprising nickel or iron (e.g., steel or stainless steel) microspheresonto a surface without melting at least one layer of the microspheres.By using HVOF and other similar thermal spray technologies, hollowmicrospheres can be quickly deposited onto surfaces. Temperatures in thespray device and microspheres can be matched so that the microspheres donot collapse, but develop an initial bond on impact with a surface ofthe target substrate.

In certain variations, where a first metal is present in a first layeralong with a second metal as part of a second layer of the hollowmicrospheres, the maximum temperature in the HVOF process may be atleast 50° C. below the melting point of the first metal, but mayapproach or exceed the melting point of the second metal on the outercoating. The second metal and/or second layer thus softens and partiallyor fully melts to enhance adhesion and bonding as the hollowmicrospheres are deposited on the substrate 110. Thus, in certainvariations, the first metal may be nickel with a melting point of about1,455° C. and the second metal may be copper having a melting point ofabout 1,084° C., so that the maximum temperature of the jetted streammay be below the melting point of nickel, but above the melting point ofcopper. In such a variation, the maximum temperature may be greater thanor equal to about 1,110° C. to less than or equal to about 1,400° C. Forexample, the temperature of the jetted stream may be above copper'smelting point of about 1,084° C. as the microspheres hit the target, butthe maximum temperature during the thermal spraying does not reach1,455° C. (the melting point of nickel). Notably, the hollowmicroparticles cool down some as they leave the gun, so higher maximumtemperatures in the gun/thermal spray device can be above 1,100° C. andcool as the hollow microparticles approach and contact the target. Wherethe second layer comprises copper and zinc, the melting pointtemperature is lower. For example, a brass alloy comprising copper andabout 32 weight % zinc has a melting point of about 903° C., thus thethermal spraying temperatures above may be adjusted accordingly. Thetemperature of the jetted stream may be near or above the brass alloy'smelting point of about 903° C. as the microspheres hit the target, butthe maximum temperature of the jetted stream during the thermal sprayingremains below 1,455° C. (the melting point of nickel).

The jetted stream is directed towards a surface of the substrate 110,where a plurality of hollow microstructures 142 is deposited in acohesive, high porosity insulating coating 144. The substrate may beformed of a variety of materials capable of withstanding hightemperatures, including metals, ceramics, and the like. In certainaspects, the substrate comprises at least one metal selected from:nickel, iron, copper, zinc, tin, aluminum, magnesium, combinations, andalloys thereof. The substrate in certain variations may include steel,superalloys, such as inconel nickel superalloys, aluminum alloys andmagnesium alloys, by way of non-limiting example.

The substrate 110 may comprise a coating or be formed of a material thatpromotes adhesion of the hollow microstructures. The surface maycomprise at least one metal selected from: copper, zinc, tin, nickel,aluminum, combinations, and alloys thereof. The surface may have ametallic surface coating or be formed of a metal comprising copperand/or zinc and/or alloys thereof. The coating comprising copper and/orzinc may be applied to any heat resistant substrate via electroplating,electroless plating, vapor deposition, flame spraying, painting, and thelike. In one variation, the substrate 110 may be formed of acopper-containing material or may comprise a copper-containing coating.Where the substrate 110 is coated, the substrate may comprise anyvariety of heat resistant materials, including steel, superalloys, suchas inconel nickel superalloys, by way of non-limiting example.

The resulting thermal spray coating comprises adjacent and/oroverlapping hollow microstructures. After thermal spraying onto thesubstrate 110, a substantial portion of the hollow microstructures 142desirably still have the enclosed void regions intact, for example,greater than about 80% of the hollow microstructures have intact voids,optionally greater than about 90%, optionally greater than about 95%,optionally greater than about 97%, and in some aspects, optionallygreater than about 98% of the voids remain intact within the hollowmicrostructures after deposition. In certain variations, the depositedhollow microstructures 142 may have the same microsphere shape as theprecursors introduced into the HVOF device 100, but it is also possiblethat they may also deform or distort into other shapes.

Accordingly, after thermal spraying the deposited microparticles mayhave some distortion in shape, but desirably retain an internal enclosedvoid region thus enhancing and retaining insulating properties of thedeposited coating. As such, an insulating coating 200 formed on asubstrate 210 via thermal spraying as shown in FIG. 4 comprises hollowmicrostructures (formed by deposition of the microspheres) with intactvoid regions. The insulating coating having such hollow microstructuresdesirably exhibits a thermal conductivity (K) of less than or equal toabout 1,000 mW/m·K at standard temperature and pressure conditions,optionally less than or equal to about 500 mW/m·K, optionally less thanor equal to about 250 mW/m·K, optionally less than or equal to about 200mW/m·K, optionally less than or equal to about 100 mW/m·K, optionallyless than or equal to about 50 mW/m·K, and in certain variations,optionally less than or equal to about 20 mW/m·K. Standard temperatureand pressure conditions are about 32° F. or 0° C. and an absolutepressure of about 1 atm or 100 KPa.

Further, the insulating coating having such hollow microstructuresdesirably exhibits a thermal capacity (c_(v)-volumetric heat capacity)of less than or equal to about 5,000 kJ/m³·K, optionally less than orequal to about 1,000 kJ/m³·K, optionally less than or equal to about 500kJ/m³·K, optionally less than or equal to about 100 kJ/m³·K, and incertain variations, optionally less than or equal to about 50 kJ/m³·K.In one variation, the insulating coating exhibits a thermal conductivity(K) of less than or equal to about 100 mW/m·K and a thermal capacity(c_(v)) of less than or equal to about 100 kJ/m³·K.

The insulating coating 200 deposited via thermal spraying, such as HVOF,may have densely packed hollow microstructures 212. In certain aspects,the insulating coating 200 comprising a plurality of hollowmicrostructures 212 deposited via thermal spraying (e.g., HVOFdeposition) has a high open porosity, for example, having a net porosityof greater than or equal to about 80 volume % of the total volume of thecoating. By net porosity, it is meant a total porosity volume includesboth a volume of void spaces within the nanostructures and a volume ofpores defined between nanostructures. In certain variations, such a netporosity is greater than or equal to about 85 volume %, optionallygreater than or equal to about 90 volume %, and in certain variations,optionally greater than or equal to about 95 volume %.

In certain variations, the insulating coating 200 may have an averagethickness of less than or equal to about 4,000 micrometers (4 mm),optionally less than or equal to about 2,000 micrometers (2 mm),optionally less than or equal to about 1,000 micrometers (1 mm),optionally less than or equal to about 500 micrometers, optionally lessthan or equal to about 400 micrometers, optionally less than or equal toabout 300 micrometers, optionally less than or equal to about 200micrometers, optionally less than or equal to about 100 micrometers,optionally less than or equal to about 75 micrometers, and in certainvariations, optionally less than or equal to about 50 micrometers. Incertain aspects, an average thickness of the insulating coating 200 isgreater than or equal to about 100 micrometers to less than or equal toabout 4,000 micrometers, optionally greater than or equal to about 100micrometers to less than or equal to about 500 micrometers, optionallygreater than or equal to about 100 micrometers to less than or equal toabout 300 micrometers. In one variation, the insulating coating has athickness of less than or equal to about 200 micrometers (μm). It shouldbe noted that the desired thickness of the coating may depend on theapplication in which the insulating coating is used, so that a thickercoating and/or coating having greater mass may be appropriate inapplications where slower thermal response is acceptable, while athinner coating or lighter coating may be selected where faster thermalresponses are desirable.

In certain aspects, the insulating layer 200 is capable of withstandingpressures of greater than or equal to about 8 MPa, optionally greaterthan or equal to about 10 MPa, optionally greater than or equal to about15 MPa, and in certain aspects, greater than or equal to about 20 MPawithout failure. With respect to high temperature performance, theinsulating layer in certain variations is configured to withstandsurface temperatures of greater than or equal to about 200° C.,optionally greater than or equal to about 250° C., optionally greaterthan or equal to about 300° C., optionally greater than or equal toabout 500° C., optionally greater than or equal to about 700° C.,optionally greater than or equal to about 1,000° C., and optionallygreater than or equal to about 1,300° C. without failure. The heatcapacity may ensure the surface of the substrate 210 on which thecoating 200 is disposed does not rise above about 250° C., for example.

As appreciated by those of skill in the art, the insulating layer 200may in fact have multiple microstructures 212 having distinctcompositions, sizes, or shapes. Such microstructures may be mixedtogether during thermal spraying or sequentially applied as distinctlayers (e.g., different compositional layers within the insulatingcoating) over one another.

With renewed reference to FIG. 1, in certain aspects, the thermalspraying may optionally be conducted at ambient conditions or at apressure of greater than or equal to about 0.5 MPa. In this manner,where a positive pressure is applied in the area surrounding the jettedstream 140 and near the substrate 110, volumetric expansion andcontraction due to changes in temperature of the hollow microspheres 102and/or deposited hollow microstructures 142 can be suppressed andminimized as the pressure is increased.

In certain alternative aspects, after the thermal spraying, thedeposited microparticles may be cooled to ambient conditions and thenfurther processed. For example, the hollow microstructures 212 in theinsulating coating 200 may be further heat treated to promote additionalbonding and sintering to enhance robustness of the coating. An exemplaryheating process for sintering may include heating a deposited layer ofmicrospheres on the substrate (having both a first metal in a firstlayer and a second metal in a second layer) to a temperature below thesolidus temperature of the second metal. For example, the second metallayer may comprise Cu or a Cu—Zn alloy. Pure Cu can thus be heated tobelow 1,084° C. (copper solidus temperature), while a Cu—Zn alloy withless than 32% by weight Zn can be heated to below about 900° C. In oneexample, sintering can be conducted at a temperature of about 800° C. inan inert atmosphere, such as argon. The heat treatment for sintering canbe conducted for greater than or equal to about 1 hour, optionallygreater than or equal to about 2 hours, optionally greater than or equalto about 4 hours, optionally greater than or equal to about 6 hours, andin certain variations, greater than or equal to about 8 hours. Inanother variation, the temperature can be raised slowly above themelting temperature of the second metal (e.g., Cu) provided all diffusedthe second metal diffused into the first metal layer (e.g., Ni) wherethe alloy including both the first metal and the second metal have ahigher melting temperature than the second metal alone.

Further, additional layers may be deposited over the insulating coating200 after deposition, for example, ceramics, nickel, vanadium,molybdenum, or other high temperature metals.

In certain variations, the substrate may be formed of a substrate thatmay have lower heat resistance, such as aluminum, which is typically notheated to temperatures above 800° C. In such an application, a surfacecoating may be disposed on the surface of the aluminum and the depositedmicroparticles may be disposed on the surface coating. The depositedmicroparticles may be heated from an exterior side, while keeping thealuminum substrate itself cool. Alternatively, an intermediatesubstrate, such as a graphite wafer having electroplated nickel, may beused. The hollow microparticles are deposited onto the nickel wafer andthen they are sintered. These materials may be added to a mold and thealuminum or other low temperature alloy may be cast around it. Yetanother variation is using an intermediate substrate, such as the nickelwafer described above. The hollow microparticles are deposited onto thenickel wafer and then they are sintered. The low temperature substratemay have a surface coating as a bonding layer, for example, an aluminumsubstrate may have a copper and/or zinc surface coating for a bondinglayer. The wafer having sintered hollow microparticles may then besintered to the piston. This secondary sintering temperature is muchless than the initial sintering temperature for the hollowmicroparticles (e.g., comprising nickel). Thus, the substrate mayoptionally comprise a nickel-containing or iron-containing sealing layerand this sealing layer could also have a fine copper or copper andnickel coating to promote bonding.

In one variation, the insulating coating may be integrated into athermal barrier composite assembly 250 as shown in FIG. 5. A thermalbarrier coating 260 is disposed on substrate 262 to define the thermalbarrier composite assembly 250. In one non-limiting embodiment, thesubstrate 262 may comprise any variety of heat resistant materials,including steel, superalloys, such as inconel nickel superalloys,aluminum alloys, and magnesium alloys, by way of non-limiting example.

The thermal barrier coating thermal barrier coating 260 includesmultiple layers (and may have more than 3 layers than those shown inFIG. 5). An optional first layer 264 (if used) is a bonding layerdisposed on a surface of the substrate 262. A second layer 270 is aninsulating layer comprising a plurality of hollow microstructures formedin accordance with certain aspects of the present disclosure. The firstlayer 264 facilitates bonding of the substrate 262 and the second layer270. A third layer 272 disposed over the second layer 270 serves as asealing layer disposed over the second layer 270. The third layer may bea thin film that is configured to resist the high temperatures andserves as a sealing layer that is impermeable to gasses and presents asmooth surface.

The optional first layer 264 that serves as a bonding layer may beformed of a metal comprising copper, or zinc, which can diffuse and bondwith the surface of substrate 262 and the second layer 270 depositedthereon, via any of the techniques described previously above. In onevariation, the first layer 264 may comprise brass, which is acopper-zinc (Cu—Zn) alloy material. Where the substrate 262 is aluminumand the microspheres comprise nickel and/or iron, in one variation,copper and zinc can be selected for inclusion in the first layer 264.Copper and zinc both have good solid solubility in aluminum, nickel, andiron, while iron and nickel have very low solid solubility in aluminum.The first layer comprising a copper and zinc alloy can be used onaluminum substrates, such as aluminum-containing pistons. Substratesformed of steel or Inconel, such as valves, can have an optional firstlayer 264 for bonding to enhance bonding of the second layer 270,although such a bonding layer may not be necessary for these substrates.Thus, a first layer 264 having copper and/or zinc provides anintermediate structural layer between the substrate 262 and the secondlayer 270 to promote diffusion bonding between the aluminum substrateand nickel or iron microstructures. It should be appreciated, however,that the substrate 262, first layer 264, and second layer 270 are notlimited to aluminum, nickel, iron and brass, but may comprise othermaterials.

The third layer 272 serves as the sealing layer disposed over theinsulating second layer 270. The sealing third layer 272 may be a hightemperature, thin film, which may be configured to withstandtemperatures of at least 1,100° C. The third layer 272 may have athickness of less than or equal to about 20 micrometers, optionally lessthan or equal to about 5 micrometers, optionally less than or equal toabout 1 micrometer. The third layer 272 is non-permeable to gases, suchas combustion gases. In this manner, the third layer 272 serves as aseal over the second layer 270. Such a seal prevents debris fromcombustion gases, such as unburned hydrocarbons, soot, partially reactedfuel, liquid fuel, and the like, from entering the openings and poresdefined between the hollow microstructures in the second layer 270.Minimizing such debris prevents gas within the pores from beingdisplaced by the debris that could cause the insulating properties to bereduced or eliminated. The third layer 272 may have a smooth outersurface, which can prevent the creation of turbulent airflow as the airflows. Further, a third layer 272 with a smooth surface can preventincreases in a heat transfer coefficient. In one non-limiting example,the third layer may be applied to the second layer 270 (after thermalspraying and cooling) via electroplating, vapor deposition, or otherapplication techniques. In one variation, the third layer 272 comprisesa heat resistant or corrosion resistant material. In one variation, thethird layer 272 may comprise molybdenum, or vanadium. The third layer272 is configured to be sufficiently resilient so as to resistfracturing or cracking during exposure to combustion gases, thermalfatigue, or debris. Further, the third layer 272 is configured to besufficiently resilient so as to withstand any expansion and/orcontraction of the underlying insulating second layer 270. The thirdlayer 272 may include multiple layers.

A thermal barrier composite assembly 250 can be used in a variety ofapplications, such as a thermal barrier on components within an internalcombustion engine. The thermal barrier composite assembly 250 may bedisposed on a face or surfaces of one or more of the components of anengine, for example, on a piston, an intake valve, an exhaust valve,interior walls of an exhaust manifold, and a combustion dome, by way ofnon-limiting example. The thermal barrier composite assembly 250 ideallyhas a low thermal conductivity to reduce heat transfer losses and a lowheat capacity so that the surface temperature of the thermal barriercomposite assembly 250 tracks the gas temperature in the combustionchamber. Thus, the thermal barrier composite assembly 250 allows surfacetemperatures of the component to swing with the gas temperatures. Thisreduces heat transfer losses without affecting the engine's breathingcapability and without causing knock. Further, heating of cool airentering the cylinder of the engine is reduced. Additionally, exhausttemperature is increased, resulting in faster catalyst light off timeand improved catalyst activity.

While the methods and materials described herein are particularlysuitable for manufacturing components of an automobile or othervehicles, they may also be used in a variety of other industries andapplications, including aerospace components, consumer goods, officeequipment and furniture, construction, industrial equipment andmachinery, farm equipment, or heavy machinery, by way of non-limitingexample. Non-limiting examples of vehicles that can incorporatecomponents prepared in accordance with certain aspects of the presentdisclosure include automobiles, trains, heavy mobile equipment,tractors, buses, motorcycles, boats, mobile homes, campers, aircraft(manned and unmanned), and tanks.

The methods and insulating coatings described herein provide lowconductivity, low heat capacity thermal barrier coatings. Such thermalbarrier coatings can improve fuel consumption and emissions for internalcombustion engines, increasing operating temperatures, while reducingafter-treatment warm up time, and improving waste heat recovery. Thedeposition methods provide the ability to deposit microspheres oncontours and various surfaces of complex parts, which otherwise may notbe possible. The insulating coatings formed by such thermal sprayingmethods exhibit only relatively low shrinkage as compared to insulatingcoatings formed of microspheres that are provided in a binder matrixthat is cured or microspheres that are sintered. Further, the insulatingcoatings formed by the thermal spraying methods are believed to haveincreased adhesion levels with the underlying substrate as compared toother methods of applying microspheres.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements or featuresof a particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure.

1. A method of forming an insulating coating comprising: thermallyspraying a jetted stream having a maximum temperature of greater than orequal to about 900° C. towards a substrate to form the insulatingcoating on the substrate, wherein the jetted stream comprises aplurality of hollow microspheres comprising a first metal layer thatcomprises a first metal selected from the group consisting of: nickel,iron, combinations, and alloys thereof and a second metal layer thatcomprises a second metal selected from the group consisting of: copper,zinc, tin, nickel, combinations, and alloys thereof, wherein the maximumtemperature of the jetted stream during the thermal spraying is at leastabout 50° C. below a melting point of the first metal layer, but greaterthan or equal to a melting point of the second metal layer, so that thesecond metal layer softens and partially or fully melts to enhanceadhesion and bonding with the substrate and the insulating coating has athermal conductivity (K) of less than or equal to about 200 mW/m·K atstandard temperature and pressure conditions.
 2. The method of claim 1,wherein the insulating coating comprises a plurality of hollowmicrostructures having intact void regions after the thermal spraying.3. The method of claim 1, wherein the insulating coating has a netporosity of greater than or equal to about 80 volume %.
 4. The method ofclaim 1, wherein the insulating coating has a thickness of less than orequal to about 200 micrometers (μm).
 5. The method of claim 1, whereinthe maximum temperature is less than or equal to about 1,400° C. 6.(canceled)
 7. The method of claim 1, wherein the substrate comprises atleast one metal selected from the group consisting of: iron, copper,zinc, tin, nickel, aluminum, combinations, and alloys thereof.
 8. Themethod of claim 1, wherein the thermal conductivity (K) is less than orequal to about 100 mW/m·K at standard temperature and pressureconditions.
 9. The method of claim 1, wherein the insulating coating hasa thermal capacity (c_(v)) of less than or equal to about 100 kJ/m³·K.10. A method of forming an insulating coating comprising: jetting astream comprising a plurality of hollow microspheres from a highvelocity oxygen fuel (HVOF) device towards an aluminum substrate havinga bonding layer comprising at least one metal selected from the groupconsisting of: copper, zinc, combinations, and alloys thereof, whereineach of the plurality of hollow microspheres comprises a first metallayer comprising nickel and a second metal layer comprising a secondmetal selected from the group consisting of: copper, zinc, combinations,and alloys thereof, wherein the stream has a maximum temperature duringthe jetting that is at least about 50° C. below a melting point of thefirst metal layer, but greater than or equal to a melting point of thesecond metal layer, so that the second metal layer softens and partiallyor fully melts to enhance adhesion and bonding with the bonding layer;and forming the insulating coating on the bonding layer disposed on thesubstrate, the insulating coating having a thermal conductivity (K) ofless than or equal to about 200 mW/m·K at standard temperature andpressure conditions.
 11. The method of claim 10, wherein the insulatingcoating comprises a plurality of hollow microstructures having intactvoid regions after the thermal spraying.
 12. The method of claim 10,wherein the insulating coating has a net porosity of greater than orequal to about 80 volume % and the insulating coating has a thickness ofless than or equal to about 200 micrometers (μm).
 13. The method ofclaim 10, wherein the maximum temperature is greater than or equal toabout 900° C. to less than or equal to about 1,400° C.
 14. (canceled)15. The method of claim 10, wherein the first metal layer comprisesnickel and the second metal layer comprises copper.
 16. The method ofclaim 10, wherein the thermal conductivity (K) is less than or equal toabout 100 mW/m·K.
 17. The method of claim 10, wherein the insulatingcoating has a thermal capacity (c_(v)) of less than or equal to about100 kJ/m³˜K.
 18. The method of claim 10, further comprising sinteringthe insulating coating after the jetting.
 19. A method of forming aninsulating coating comprising: jetting a stream comprising a pluralityof hollow microspheres from a high velocity oxygen fuel (HVOF) devicetowards a substrate to form a layer of deposited hollow microstructures,wherein the stream has a maximum temperature during the jetting that isgreater than or equal to about 900° C. to less than or equal to about1400° C. and a supersonic velocity of greater than or equal to about 343m/s and less than or equal to about 400 m/s, wherein each of theplurality of hollow microspheres comprises a first metal layercomprising a first metal selected from the group consisting of: nickel,iron, combinations, and alloys thereof and a second metal layercomprising a second metal selected from the group consisting of: copper,zinc, tin, nickel, combinations, and alloys thereof, wherein the streamhas a maximum temperature during the jetting that is at least about 50°C. below a melting point of the first metal layer, but greater than orequal to a melting point of the second metal layer, so that the secondmetal layer softens and partially or fully melts to enhance adhesion andbonding with the substrate; and sintering the layer of deposited hollowmicrostructures to form the insulating coating on the substrate having athermal conductivity (K) of less than or equal to about 200 mW/m·K atstandard temperature and pressure conditions and a thermal capacity(c_(v)) of less than or equal to about 100 kJ/m³·K.
 20. The method ofclaim 19, wherein the sintering occurs by heating the deposited hollowmicrostructures to a temperature of greater than or equal to about 800°C. for greater than or equal to about 8 hours.